It is known that the buildup of deposits and contaminants on the surface of heat exchangers reduces their efficiency, and that the removal of these deposits restores efficiency.
In the field of refrigeration and chillers, the evaporator heat exchanger is a large structure, containing a plurality of parallel tubes, within a larger vessel comprising a shell, through which refrigerant flows, absorbing heat and evaporating. Outside the tubes, an aqueous medium, such as brine, circulates and is cooled, which is then pumped to the process region to be cooled. Such an evaporator may hold hundreds or thousands of gallons of aqueous medium with an even larger circulating volume. The known process for cleaning the aqueous portion these heat exchangers involves flushing an aqueous cleaning fluid around the heat exchange pipes, hoping to dissolve or dislodge deposits. More aggressive cleaning involves dismantling the shell of the evaporator and manually cleaning the refrigerant tubes by scrubbing. This cleaning process is thus cumbersome and inefficient.
U.S. Pat. Nos. 4,437,322; 4,858,681; 5,653,282; 4,539,940; 4,972,805; 4,382,467; 4,365,487; 5,479,783; 4,244,749; 4,750,547; 4,645,542; 5,031,410; 5,692,381; 4,071,078; 4,033,407; 5,190,664; and 4,747,449 relate to heat exchangers and the like.
The operation of various pipes and tubes and vessels including heat exchangers is routinely impeded by the buildup of sedimentation in and around heat exchange surfaces and components causing restriction of flow and impediment of enthalpy or both. Devices using acoustic-type energy to resist or remove sedimentation have been suggested. In such devices, a portion of energy is imparted to tubes and other walls encountered and to molecules and particles in suspension or solution in the fluid. If the imparted energy density is less than the deposition energy of suspended or dissolved particles and/or the binding energy of deposited particles, deposition restrain and/or dislodgment of sediment particles will be less efficient in accordance with the laws of statistics. If the imparted energy density exceeds such sedimentation rate and/or binding energy, sedimentation will be prevented and existing sediment more rapidly dissipated.
The issue then becomes effectively and efficiently imparting the acoustic waves. The efficiency of prior art acoustic devices is limited, and, moreover, there is a limit to the power which can be applied to the transducer because of the so-called cavitation effect in the fluid and risk of damage. While composite wave devices have been suggested, these utilize resonance effects and produce resultant standing wave patterns which may leave areas untreated and subject to load and configuration variances.
U.S. Pat. Nos. 2,987,068; 3,640,295; and 3,295,596, expressly incorporated herein by reference, as well as British Pat. Nos. 1,456,664 and 1,385,750 each teach ultrasonic cleaning apparatuses which include a plurality of transducers affixed to a cleaning vessel or container for effecting ultrasonic cleaning of items inserted within the vessel or container. U.S. Pat. No. 3,240,963, expressly incorporated herein by reference, teaches a plurality of transducers movably mounted within a vessel for cleaning items disposed therein. Ultrasonic transducers are shown in U.S. Pat. No. 2,716,708, expressly incorporated herein by reference, and British Pat. No. 1,282,552. U. S. Pat. No. 3,371,233 discloses a multifrequency ultrasonic cleaning apparatus. U. S. Pat. No. 3,638,087, expressly incorporated herein by reference, discloses a gated power supply for sonic cleaners.
High pressure, low-frequency shock waves are used to unplug blocked pipes (Simon, U.S. Pat. No. 4,974,617, expressly incorporated herein by reference; Coon et al., U.S. Pat. No. 4,551,041, expressly incorporated herein by reference), and clean corrosion products and sedimentation from the interior walls of heat exchanger tubes (Scharton et al., U.S. Pat. No. 4,645,542, expressly incorporated herein by reference). Such techniques are, however, not suitable for cleaning the interior surfaces of elongated tubes for the purpose of degreasing or cleaning, due to the high pressures (up to 5,000 psi) of the shock waves and extended time periods required (1-24 hours).
It is known in the ultrasonic cleaning art that high peak or power bursts are necessary for aggressive cleaning or for cavitating liquids. The prior art provides various power burst controls for adjusting a duty cycle, amplitude and frequency of the transducer output, in addition to the pulse sequences and parameters.
U.S. Pat. No. 4,736,130, expressly incorporated herein by reference, relates to a multiparameter generator for ultrasonic transducers, which controls seven variables. These are: 1) the time duration of a power pulse train, which is followed by a 2) time period of no activity for degassing, 3) the time duration of individual power bursts during the power train period, 4) the time duration of periods of no activity between the individual power bursts, 5) the range of amplitude modulation of each power burst, 6) the mean transmitted frequency, and 7) a frequency modulation index.
In U.S. Pat. No. 4,398,925 Trinh et al., expressly incorporated herein by reference, relates to an ultrasonic transmitting apparatus for removing dissolved gas in a fluid. It is disclosed that the transmitted frequency is swept from 0.5 kHz to 40 kHz and that the ratio between the low and high frequency limit should be at least 10 times.
In U.S. Pat. Nos. 3,648,188, and 4,588,917, expressly incorporated herein by reference, relate to power oscillators with different resonant arrangements and positive feedback components to cause oscillation.
U.S. Pat. No. 4,864,547, expressly incorporated herein by reference, relates to a system for producing a soft start and means to vary the power to the transducer.
Several phase locked loop arrangements are described so that a resonant frequency of the transducer is locked onto by the drive electronics. U.S. Pat. No. 4,748,365, expressly incorporated herein by reference, is an example of this which describes means for searching for the load resonance point and then locking onto it.
U.S. Pat. No. 4,120,699, expressly incorporated herein by reference, relates to a method for acoustical cleaning of heat exchangers and the like.
U.S. Pat. Nos. 4,244,749 and 4,375,991, expressly incorporated herein by reference, relate to ultrasonic cleaning methods for heat exchangers.
U.S. Pat. No. 4,358,204, expressly incorporated herein by reference, relates to an ultrasonic method for cleaning ultraviolet lamps in a treatment chamber.
U.S. Pat. No. 4,366,003, expressly incorporated herein by reference, relates to a method for periodically cleaning out solid deposits from heat exchanger pipes.
U.S. Pat. No. 4,645,543, expressly incorporated herein by reference, relates to a method of pulse pressure cleaning the interior of heat exchanger tubes.
U.S. Pat. No. 4,750,547, expressly incorporated herein by reference, relates to a method for cleaning inner surfaces of heat-transfer tubes in a heat exchanger employing ultrasonic waves.
U.S. Pat. No. 4,773,357, expressly incorporated herein by reference, relates to a water cannon apparatus for cleaning tube bundle heat exchangers.
U.S. Pat. No. 4,974,617, expressly incorporated herein by reference, relates to a low frequency sonic method for clearing a liquid-filled pipe.
U.S. Pat. No. 4,991,609, expressly incorporated herein by reference, relates to an ultrasonic cleaning method.
U.S. Pat. No. 4,966,177, expressly incorporated herein by reference, relates to a method for ultrasonic cleaning of fuel rod tubes.
U.S. Pat. No. 4,972,805, expressly incorporated herein by reference, relates to a gas-pulse method and apparatus for removing foreign matter from heat exchanger tubesheets.
U.S. Pat. No. 5,076,854, expressly incorporated herein by reference, relates to a multifrequency hopping method for cleaning.
U.S. Pat. No. 5,109,174, expressly incorporated herein by reference, relates to an ultrasonic generator for a cleaning system.
U.S. Pat. No. 5,137,580, expressly incorporated herein by reference, relates to a alternating multifrequency ultrasonic cleaning system.
U.S. Pat. Nos. 5,289,838 and 5,529,635, expressly incorporated herein by reference, relate to methods for ultrasonically cleaning interior surfaces, for example, of tubes.
U.S. Pat. No. 5,339,844, expressly incorporated herein by reference, relates to a method for ultrasonic cleaning in liquid CO.sub.2.
U.S. Pat. No. 5,413,168, expressly incorporated herein by reference, relates to a solvent-based method for cleaning heat exchangers.
U.S. Pat. No. 5,458,860, expressly incorporated herein by reference, relates to a sonic and solvent-based cleaning method.
U.S. Pat. No. 5,462,604, expressly incorporated herein by reference, relates to a method of driving an ultrasonic transducer for cleaning.
U.S. Pat. No. 5,467,791, expressly incorporated herein by reference, relates to an ultrasonic cleaning method.
U.S. Pat. No. 5,496,411, expressly incorporated herein by reference, relates to an ultrasonic vibration generator.
U.S. Pat. Nos. 5,711,327, 4,705,054 and 4,372,787, expressly incorporated herein by reference, relate to ultrasonic methods for cleaning radiators and the like.
U.S. Pat. No. 5,777,860, expressly incorporated herein by reference, relates to an ultrasonic frequency power supply.
The use of ultrasonics to enhance the cleaning effectiveness of solvents is well known. Ultrasonic techniques are particularly valuable when aqueous solvents are used, since aqueous solvents are intrinsically less effective than CFC. solvents. The object to be cleaned is placed in a bath containing a mixture of water or some other solvent. Ultrasonic waves agitate the mixture, inducing cavitation at sites where the localized pressure is low enough that the fluid can no longer support the sound wave. At typical ultrasonic frequencies, cavitation occurs at sound pressures of approximately 0.36 watt/cm.sup.2 in water. The mechanical disruption and agitation of the fluid at the cavitation sites significantly enhances its effectiveness as a cleaner and degreaser.
While it is known to apply ultrasonic cleaning techniques to certain types of heat exchangers, such as coolers for nuclear power plants, these techniques are inapplicable to heating ventilation and air conditioning (HVAC) and process chillers, as access to the evaporator tubes is poor and the physical dimensions are smaller. Further, the biofouling of nuclear power plant heat exchangers is qualitatively different from the mineral and agglomerate deposits on the chiller tubes. Finally, the chiller evaporator is less robust than a power plant heat exchanger, and thus is not amenable to strenuous cleaning methods. Thus, any cleaning method which substantially risks damage to the chiller evaporator tubing is unacceptable.
In order to understand the mechanics of ultrasonics, it is necessary to first have a basic understanding of sound waves, how they are generated and how they travel through a conducting medium. The dictionary defines sound as the transmission of vibration through an elastic medium which may be a solid, liquid, or a gas. A sound wave is produced when a solitary or repeating displacement is generated in a sound conducting medium, such as by a "shock" event or "vibratory" movement. The displacement of air by the cone of a radio speaker is a good example of "vibratory" sound waves generated by mechanical movement. As the speaker cone moves back and forth, the air in front of the cone is alternately compressed and rarefied to produce sound waves, which travel through the air until they are finally dissipated. There are also sound waves which are created by a single "shock" event. An example is thunder which is generated as air instantaneously changes volume as a result of an electrical discharge (lightning). Another example of a shock event might be the sound created as a wooden board falls with its face against a cement floor. Shock events are sources of a single compression wave which radiates from the source and may also include a bulk movement component.
In elastic media such as air and most solids, there is a continuous transition as a sound wave is transmitted. In non-elastic media such as water and most liquids, there is continuous transition as long as the amplitude or "loudness" of the sound is relatively low. As amplitude is increased, however, the magnitude of the negative pressure in the areas of rarefaction eventually becomes sufficient to cause the liquid to fracture because of the negative pressure, causing a phenomenon known as cavitation. Cavitation "bubbles" are created at sites of rarefaction as the liquid fractures or tears because of the negative pressure of the sound wave in the liquid. As the wave fronts pass, the cavitation "bubbles" oscillate under the influence of positive pressure, eventually growing to an unstable size. Finally, the violent collapse of the cavitation "bubbles" results in implosions, which cause shock waves to be radiated from the sites of the collapse and are also associated with "jets" of medium. The collapse and implosion of myriad cavitation "bubbles"; throughout an ultrasonically activated liquid result in the effect commonly associated with ultrasonics. It has been calculated that temperatures in excess of 10,000.degree. F. (or about 5,000.degree. C.) and pressures in excess of 10,000 PSI (or about 500 atm) are generated at the implosion sites of cavitation bubbles.
Because of the very short duration of the bubble expansion and collapse cycle, the liquid surrounding the bubble quickly absorbs the heat and the area cools quickly. As a result, the tank and liquid becomes only warm and does not heat up due to the introduction of parts during the cleaning process. Effectively, in an ultrasonic cleaning system, the ultrasonic energy is concentrated near surfaces or discontinuities in the path of the sonic wave, resulting in interference between the incident and reflected portions of the wave.
The implosion event, when it occurs near a hard surface, changes the bubble into a jet about one-tenth the bubble size, which travels at speeds up to 400 km/hr toward the hard surface. With the combination of pressure, temperature, and velocity, the jet frees contaminants from their bonds with the substrate. Because of the inherently small size of the jet and the relatively large energy, ultrasonic cleaning has the ability to reach into small crevices and remove entrapped soils very effectively.
Cavitation and implosion as a result of ultrasonic activity displace and remove loosely held contaminants such as dust from surfaces. For this to be effective, it is necessary that the coupling medium be capable of wetting the particles to be removed.
Some contaminants are comprised of insoluble particles loosely attached and held in place by ionic or cohesive forces. These particles need only be displaced sufficiently to break the attractive forces to be removed.
Contaminations can also, of course, be more complex in nature, consisting of combination soils made up of both soluble and insoluble components. The effect of ultrasonics is substantially the same in these cases, as the mechanical micro-agitation helps speed both the dissolution of soluble contaminants and the displacement of insoluble particles. Ultrasonic activity has also been demonstrated to speed or enhance the effect of many chemical reactions. This is probably caused mostly by the high energy levels created as high pressures and temperatures are created at the implosion sites. It is likely that the superior results achieved in many ultrasonic cleaning operations may be at least partially attributed to the sonochemistry effect.
In the field of sonic cleaning, the range of useful frequencies extends from sonic and ultrasonic (above 18 kHz to about 100 kHz) to megasonic (500 kHz to 1 MHz and beyond). Typically, the ultrasonic systems are employed for gross cleaning, while megasonic systems are employed for fine cleaning or cleaning of delicate parts. See, Beck, Mark and Venerbeck, Richard B., "Megasonics Help `Stream`-line Sensitive Substrate Cleaning", Precision Cleaning, January, 1998, pp. 15-19.
Acoustic cavitation is generally regarded as the principle mechanism of particle removal in acoustic cleaning. In an acoustic field, a bubble or cavity in the liquid can be created when the liquid pressure momentarily drops below the vapor pressure as a result of pressure oscillation. There are four methods of producing cavitation.
The pressure oscillations which produce acoustic cavitation cause bubbles to contract and expand. Gas from the liquid diffuses into the bubble upon expansion, and leaves the bubble during contraction. When the bubble reaches a size that can no longer be sustained by its surface tension, the bubble will collapse and the intensity of this collapse on a substrate surface is related to the type of acoustic cavitation produced. It is noted that near the point of collapse, there is a non-linearity, which may be explained by physical effects. In order to form a gas, a heat of vaporization must be added. When the bubble collapses, the latent heat of vaporization is released. Thus, ultrasonic cleaning depends on cavitation of the liquid media with ultimate collapse of the bubbles, which release shock waves, and small jets of media atoms. Depending on the proximity of the cavitation to the surface, the cleaning effect is either by the vibrations, amplified by the cavitation effect and conducted by the medium, or directly by the jets involved in the cavitation.
During the negative pressure portion of the sound wave, the liquid is thus torn apart and cavitation bubbles start to form. As a negative pressure develops within the bubble, gasses dissolved in the cavitating liquid start to diffuse across the boundary into the bubble. As negative pressure is reduced due to the passing of the rarefaction portion of the sound wave and atmospheric pressure is reached, the cavitation bubble starts to collapse due to its own surface tension. During the compression portion of the sound wave, any gas which diffused into the bubble is compressed and finally starts to diffuse across the boundary again to re-enter the liquid. This process, however, is never complete as long as the bubble contains gas since the diffusion out of the bubble does not start until the bubble is compressed. And once the bubble is compressed, the boundary surface available for diffusion is reduced. As a result, cavitation bubbles formed in liquids containing (noncondensable) gas do not collapse all the way to implosion but rather result in a small pocket of compressed gas in the liquid. This phenomenon can be useful in degassing liquids. The small gas bubbles group together until they finally become sufficiently buoyant to come to the surface of the liquid.
Liquids containing dissolved gas thus have suppressed cavitation intensity, because the gas diffuses into cavitation bubbles formed during the negative pressure portion of the sound wave, and cushions the implosion of the bubble during the positive portion of the wave. As a result, there is no violent implosion. Dissolved gas can be eliminated from liquids by applying ultrasonic energy intermittently, or by heating the liquid. During intermittent excitation, gas bubbles will form as the energy is applied and then float to the surface when it is turned off. As temperature is increased, liquids are able to hold less dissolved gas. It is a good idea to fill a cleaning tank with liquid that is at or near the operating temperature when possible.
There are two types of acoustic cavitation: transient and stable (or controlled). Transient cavities exist for a few cycles, and are followed by a rapid and violent collapse, or implosion, that produces very high local temperatures. Ultrasonic cleaning frequencies, typically between 20 and 350 kHz, transform low-energy/density sound waves into high-energy/density collapsing bubbles, producing transient acoustic cavitation. Transient acoustic cavitation can cause damaging surface erosion in more sensitive substrates. In routine cleaning operations of uncontrolled mechanisms and under poorly controlled conditions, such damage would be very undesirable.
Megasonic cleaning systems typically use transducers exploiting the piezoelectric effect at high frequencies between 700 and 1000 kHz to remove submicron particles from substrates. Cleaning is accomplished by exciting a ceramic piezoelectric crystal with a high-frequency AC voltage, causing the ceramic material to change dimension, or vibrate. These vibrations are transmitted by the ceramic transducer to produce megasonic waves in the cleaning fluid. Megasonic frequencies are typically exploited to produce stable acoustic cavitation, which is characterized by mostly small, gas-filled cavities. Stable cavitation bubbles have less time to grow and are smaller, resulting in a less vigorous collapse than in transient cavitation. The implosion associated with these smaller, gas-filled bubbles is less likely to produce surface damage. Thus, megasonic cavitation is better suited for sensitive substrate surfaces, but is less effective in cleaning grossly contaminated surfaces.
Ultrasonics simultaneously cleans all sides of a submerged part, while megasonics cleans only the surfaces of the part facing the acoustic stream formed by the piezoelectric crystal. This is due to the highly directional nature of the megasonic waves and their absorption by the media, resulting in line-of-sight action.
In an acoustic cleaning system working in continuous mode, sound waves are reflected from substrate surfaces, exterior containment walls, and the free surface (is any) of the liquid medium. The pressure amplitude, or sonic power, required to achieve controlled cavitation and acoustic streaming depends on pulse width, dissolved gas content in the cleaning fluid, and power input. The threshold pressure needed to initiate cavitation has been found to be a strong function of the pulse width and the duty cycle of the power input into the transducer. The increase of cavitation threshold pressure with a decrease in pulse width may be related to the time needed for a bubble to grow by rectified diffusion. With short pulses, bubbles may not have enough time to grow transient cavities. Megasonics cleaning, therefore, is optimized by pulsing the input power, thus providing effective particle removal and enhanced control over cavitation.
Acoustic streaming is the time-independent fluid motion generated by a sound field. This motion, caused by the loss of acoustic momentum by attenuation or absorption of a sound beam, enhances particle dissolution and transports detached particles away from surfaces, decreasing particle redeposition. Since the absorption coefficients for high (megasonic) frequency sonic waves in a liquid is much greater than low (ultrasonic) frequency sonic waves, streaming is a quantitatively more important effect in megasonic systems.
Cleaning activity depends not only on the local sound intensity at the substrate surface, but also on the bulk motion of the fluid, which carries removed particles away from substrates and reduces the surface boundary concentration of dissolved contaminants. In a closed environment, bulk motion is produced by acoustic streaming. Stable cavitation bubbles also influence the bulk flow through buoyancy forces and microscopic flow through acoustic streaming.
Fluid velocity in a stream is a function of the velocity of the fluid produced by acoustic waves, and the velocity of acoustic streaming. Pressure is also divided into two parts: the acoustic pressure generated by acoustic waves and the hydraulic pressure caused by acoustic streaming.
The acoustic waves used in sonic cleaning may either slide, roll, or lift a particle from its initial position on a substrate, depending on the size and shape of the particle, as well as the nature of the hydrodynamic force being applied.
In varying degrees, limited frequency sweep has always been inherent in the operation of ultrasonic cleaning equipment. Variations in liquid level, solution temperature and workload configuration tend to de-tune the system and, for this reason, ultrasonic generators have incorporated feedback circuits of one sort or another to neutralize the effect of these variables. These same feedback circuits or loops have also served to allow the generator to compensate for minor variations in the resonant frequencies of individual transducers within a given tank assembly. See, Layton, Howard M., "Ultrasonic Frequencies Make a Clean Sweep", Precision Cleaning, January 1998, pp. 9-14.
There are seven major concerns related to successful ultrasonic cleaning: time, temperature, chemistry, proximity of surface to be cleaned to the transducer, ultrasonic output frequency, watts per gallon, and loading of the cleaning system.
It is believed that high frequencies penetrate more and lower frequencies are more aggressive. The majority of the ultrasonic cleaning that is done in industrial applications uses 40 kHz as the base frequency. Lower frequencies, such as 20-25 kHz, are used for large masses of metal where ultrasonic erosion is of little consequence. The large metal mass dampens or absorbs a great amount of the ultrasonic cleaning power.
Most industrial ultrasonic cleaning systems use watt density from 50-100 Watts per gallon. However, there is what is known as "the large tank phenomenon", which indicates that fluid volumes over 50 gallons usually require only about 20 watts per gallon.
Maximizing cavitation of the cleaning liquid is obviously very important to the success of the ultrasonic cleaning process. Several variables affect cavitation intensity. Temperature is the most important single parameter to be considered in maximizing cavitation intensity. This is because so many liquid properties affecting cavitation intensity are related to temperature. Changes in temperature result in changes in viscosity, the solubility of gas in the liquid, the diffusion rate of dissolved gasses in the liquid, and vapor pressure, all of which affect cavitation intensity. In pure water, the cavitation effect is maximized at approximately 160.degree. F.
Temperature and chemistry are closely related. The operating temperature should be at least about 6.degree. F. below the boiling point of the liquid, although other considerations control the operating temperature. The containment pressure may be varied to control cavitation effects as well.
In general, liquids with higher surface tension exhibit higher cavitation intensities. This is thought to be because the higher surface tension results in greater energy being released as cavitation bubbles implode. More viscous liquids require more energy to cavitate. As viscosity is increased (perhaps to that of motor oil) ultrasonic cavitation is no longer possible using normal ultrasonic techniques.
The viscosity of a liquid may thus be minimized for increased cavitation effect. Viscous liquids are sluggish and cannot respond quickly enough to form cavitation bubbles and violent implosion. The viscosity of most liquids is reduced as temperature is increased.
For most effective cavitation, the cleaning liquid must contain as little dissolved gas as possible. Gas dissolved in the liquid is released during the bubble growth phase of cavitation and prevents its violent implosion which is required for the desired ultrasonic effect. The amount of dissolved gas in a liquid is reduced as the liquid-temperature is increased.
The diffusion rate of dissolved gasses in a liquid is increased at higher temperatures. This means that liquids at higher temperatures give up dissolved gasses more readily than those at lower temperatures, which aids in minimizing the amount of dissolved gas in the liquid.
A moderate increase in the temperature of a liquid brings it closer to its vapor pressure, meaning that vaporous cavitation is more easily achieved. Vaporous cavitation, in which the cavitation bubbles are filled with the vapor of the cavitating liquid, is the most effective form of cavitation. As the boiling temperature is approached, however, the cavitation intensity is reduced as the liquid starts to boil at the cavitation sites and at the transducers.
Cavitation intensity is directly related to Ultrasonic Power at the power levels generally used in ultrasonic cleaning systems. As power is increased substantially above the cavitation threshold, cavitation intensity levels off and can only be further increased through the use of focusing techniques. Therefore, acoustic lenses and reflectors and phased array transducers may be employed.
Cavitation intensity is inversely related to Ultrasonic Frequency. As the ultrasonic frequency is increased, cavitation intensity is reduced because of the smaller size of the cavitation bubbles and their resultant less violent implosion. The reduction in cavitation effect at higher frequencies may be overcome by increasing the ultrasonic power.
As ultrasonic frequency is increased, more power must be applied to maintain the same cavitation intensity. This is because at higher frequencies, relatively fewer sites are present which can become nuclei for cavitation bubbles. The higher the frequency, the smaller the nucleus for cavitation must be. Fewer cavitation bubbles of a smaller average size result in less cavitation intensity overall. Most ultrasonic cleaning equipment operates at frequencies between 21 and 45 kHz. Although a variation of frequency within this relatively narrow range seldom has a dramatic effect on cleaning, it may occasionally be considered as a variable in achieving maximum cleaning. Cases where it may be important are these where every small area must be penetrated and where the parts being cleaned may be frequency sensitive.
Various effects are produced by changing the speed and magnitude of a frequency modulation of the acoustic wave. The frequency may be modulated from once every several seconds to several hundred times per second with the magnitude of variation ranging from several hertz to several kilohertz for ultrasonic waves and correspondingly increased modulation for megasonic waves. Sweep may be used to prevent damage to delicate parts or to reduce the effects of standing waves. A combination of sweep and pulse operation may also be found especially useful in facilitating the cavitation of various organic solvents. Frequency hopping according to a random or pseudorandom pattern or other techniques to provide varying interference patterns to assure complete surface treatment may be employed.
The percentage of time that the ultrasonic energy is on may also be changed to produce varied results. At slower pulse rates, more rapid degassing of liquids occurs as coalescing bubbles of air are given an opportunity to rise to the surface of the liquid during the time the ultrasonic energy is off. At more rapid pulse rates the cleaning process may be enhanced as repeated high energy "bursts" of ultrasonic energy occur each time the energy source is turned on.
Various effects are produced by changing the speed and magnitude of the frequency modulation. The frequency may be modulated from once every several seconds to several hundred times per second with the magnitude of variation ranging from several hertz to several kilohertz. Sweep may be used to prevent damage to extremely delicate parts or to reduce the effects of standing waves in cleaning tanks.
In order to produce the positive and negative pressure waves in the medium, a mechanical vibrating device is required. Typical ultrasonic manufacturers make use of a diaphragm attached to high-frequency transducers. The transducers, which vibrate at their resonant frequency due to a high-frequency electronic generator source, induce amplified vibration of the diaphragm. This amplified vibration is the source of positive and negative pressure waves that propagate through the liquid medium.
There are two types of ultrasonic transducers used in the industry, piezoelectric and magnetostrictive. Both have the same functional objective, but the two types have dramatically different performance characteristics.
Piezoelectric transducers are made up of several components. The ceramic (usually lead zirconate) crystal is sandwiched between two strips of tin. The ultrasonic transducers preferably operate between 18 kHz and 80 kHz. Other suitable piezoelectric transducer materials include lithium niobate, lithium tantalate, barium sodium niobate, bismuth germanate, lead titanate zirconate, and barium titanate. When voltage is applied across the strips it creates a displacement in the crystal, known as the piezoelectric effect. When these transducers are mounted to a diaphragm (wall or bottom of the tank), the displacement in the crystal causes a movement of the diaphragm, which in turn causes a pressure wave to be transmitted through the liquid medium in the tank. Because the mass of the crystal is not well matched to the mass of the stainless steel diaphragm, an intermediate aluminum block is used to improve impedance matching for more efficient transmission of vibratory energy to the diaphragm. The assembly is inexpensive to manufacture due to low material and labor costs. This low cost makes piezoelectric technology desirable for ultrasonic cleaning. However, piezoelectric transducers have several shortcomings.
The most common problem is that the performance of a piezoelectric unit deteriorates over time. This can occur for several reasons. The crystal tends to depolarize itself over time and with use, which causes a substantial reduction in the strain characteristics of the crystal. As the crystal itself expands less, it cannot displace the diaphragm as much. Less vibratory energy is produced, with a corresponding decrease in cavitation. Additionally, piezoelectric transducers are often mounted with an epoxy adhesive, which is subject to fatigue at the high frequencies and high heat generated by the transducer and solution. The epoxy bond eventually loosens, rendering the transducer useless. The capacitance of the crystal also changes over time and with use, affecting the resonant frequency and causing the generator to be out of tune with the crystal resonant circuit.
Although the piezoelectric transducers utilize an aluminum block insert to improve impedance matching (and therefore energy transfer into the radiating diaphragm), they still have relatively low mass. This low mass limits the amount of energy transfer into the medium (as can be seen from the basic equation for kinetic energy, e=1/2 mv.sup.2). Due to the low mass of the piezoelectric transducers, a thin diaphragm must be used. A thick plate simply will not flex (and therefore cause a pressure wave) given the relatively low energy output of the piezoelectric transducer. However, there are several problems with using a thin diaphragm. A thin diaphragm driven at a certain frequency tends to oscillate at the upper harmonic frequencies as well, which creates smaller implosions. Another problem is that cavitation erosion, a common occurrence in ultrasonic cleaners, can wear through a thin-wall diaphragm. Once the diaphragm is penetrated, the solution will damage the transducers and wiring, leaving the unit useless and requiring major repair expense.
Magnetostrictive Transducers are known for their ruggedness and durability in industrial applications. Zero-space magnetostrictive transducers consist of nickel laminations attached tightly together with an electrical coil placed over the nickel stack. When current flows through the coil it creates a magnetic field. This is analogous to deformation of a piezoelectric crystal when it is subjected to voltage. When an alternating current is sent through the magnetostrictive coil, the stack vibrates at the frequency of the current.
The nickel stack of the magnetostrictive transducer is silver brazed directly to the resonating diaphragm. This has several advantages over an epoxy bond. The silver braze creates a solid metallic joint between the transducer and the diaphragm that will never loosen. The silver braze also efficiently couples the transducer and the diaphragm together, eliminating the damping effect that an epoxy bond creates. The use of nickel in the transducers means there will be no degradation of the transducers over time; nickel maintains its magnetostrictive properties on a constant level throughout the lifetime of the unit. Magnetostrictive transducers also provide more mass, which is a major factor in the transmission of energy into the solution in the ultrasonic tank. Zero-space magnetostrictive transducers have more mass than piezoelectric transducers, so they drive more power into the medium, and this makes them less load-sensitive than piezoelectric systems.
Magnetostrictive transducers utilize the principle of magnetostriction in which certain materials expand and contract when placed in an alternating magnetic field. Alternating electrical energy from the ultrasonic generator is first converted into an alternating magnetic field through the use of a coil of wire. The alternating magnetic field is then used to induce mechanical vibrations at the ultrasonic frequency in resonant strips of nickel or other magnetostrictive material which are attached to the surface to be vibrated. Because magnetostrictive materials behave identically to a magnetic field of either polarity, the frequency of the electrical energy applied to the transducer is 1/2 of the desired output frequency. Magnetostrictive transducers were first to supply a robust source of ultrasonic vibrations for high power applications such as ultrasonic cleaning.
Because of inherent mechanical constraints on the physical size of the hardware as well as electrical and magnetic complications, high power magnetostrictive transducers seldom operate at frequencies much above 20 kilohertz. Piezoelectric transducers, on the other hand, can easily operate well into the megahertz range. Magnetostrictive transducers are generally less efficient than their piezoelectric counterparts. This is due primarily to the fact that the magnetostrictive transducer requires a dual energy conversion from electrical to magnetic and then from magnetic to mechanical. Some efficiency is lost in each conversion. Magnetic hysteresis effects also detract from the efficiency of the magnetostrictive transducer.
A radiating diaphragm that uses zero-space magnetostrictive transducers is usually 5 mm (3/16 in.) or greater in thickness, eliminating any chance for cavitation erosion wearthrough. Heavy nickel stacks can drive a plate of this thickness and still get excellent pressure wave transmission into the aqueous solution.
The magnetostrictive transducer is not as efficient as a piezoelectric transducer. That is, for a given voltage or current displacement, the piezoelectric transducer will exhibit more deflection than the magnetostrictive transducer. However, the efficiency of concern should be that of the entire transducing system, including not only the transducer but also the elements that make up the transducer, as well as the diaphragm. It is the inferior mounting and impedance matching of a piezoelectric-driven diaphragm that reduces its overall transducing efficiency relative to that of a magnetostrictive transducer.
The ultrasonic generator converts a standard electrical frequency of, e.g., 50 or 60 Hz into the high frequencies required in ultrasonic transmission, generally in the range of 18 to 80 kHz, but which may extend from sonic frequencies, especially in combination with ultrasonic frequencies, to about 100 kHz. Many of the better generators today use advanced technologies such as sweep frequency, harmonic generation, and autofollow circuitry. Frequency sweep circuitry drives the transducers between a bandwidth slightly greater and slightly less than the center frequency. For example, a transducer designed to run at 30 kHz will be driven by a generator that sweeps between 29 and 31 kHz. This technology eliminates the standing waves and hot spots in the tank that are characteristic of older, fixed-frequency generators. Autofollow circuitry is designed to maintain the center frequency when the medium is subjected to varying load conditions. With autofollow circuitry, the generator matches electrically with the mechanical load, providing optimum output at all times to the ultrasonic transducer.
See, Ultrasonic Cleaning, Tool and Manufacturing Engineers Handbook, Vol. 3, Materials, Finishing, and Coating, C. Wick and R. F. Veilleux, Ed., Society of Manufacturing Engineers, 1985, p 18-20 to 18-24; F. J. Fuchs, Ultrasonic Cleaning, Metal Finishing Guidebook and Directory, Elsevier Science, 1992, p 134-139; See, "Ultrasonic Cleaning", published in the ASM Handbook, Vol. 5, Surface Engincering, p 44-47, copyright 1994, ASM International, Materials Park, Ohio 44073-0002. See also www.upcorp.com/explanation; www.ij.net/GCU/tech.html; www.bluewaveinc.com/reprint.htm; www.caeblackstone.com/contents.html (and linked pages). See also www.grecobrothers.com/hpdg.htm.
Applying a square wave signal to an ultrasonic transducer results in an acoustic output rich in harmonics. The result is a multi-frequency cleaning system which vibrates simultaneously at several frequencies which are harmonics of the fundamental frequency. Multi-frequency operation offers the benefits of all frequencies combined, although the acoustic power is spread over a wide band.
Basically, the cavitation threshold I can be determined by the following formula: EQU Ic=[(0.707).times.10.sup.6 P.sub.c ].sup.2.times.10.sup.-7 =0.3 P.sub.c Watt/cm.sup.2 /.rho.C
Where P.sub.c equals the peak pressure of sound wave causing cavitation per atmosphere, where .rho. equals one gram/cm.sup.3, and C equals 1.5.times.10.sup.5 cm/second. Therefore, a cavitation threshold at one atmosphere is equivalent to a plane wave intensity of 0.3 watts per cm.sup.2. With 0.3 watts/cm.sup.2 being the plane wave threshold, the desired power level radiated from the ultrasonic cleaning apparatus would be between 0.5 to 2 watts cm.sup.2 to insure that cavitation is taking place. It is interesting to note that pressure increases the effectiveness of ultrasonic cleaning up to 7 or 8 atmospheres. As a result, the farther down the ultrasonic cleaning apparatus 10 is employed, the more effective the cleaning will be, quite the converse of the cleaning problems which are encountered through the use of mechanical scraping or brushing. In fact, if the pressure is increased the power level under certain circumstances can be reduced.